Human tissues can absorb many forms of energy including heat, sound, and light. Blood is one tissue that contains many absorbers of light energy. The hemoglobin molecules in the blood have extensively characterized spectrophotometric signatures and can be identified by the strength and wavelength of their absorptions. Contemporary clinical laboratory medicine relies on these physical properties of light energy absorption for the routine measurement of bilirubin, oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), methemoglobin (metHb), carboxyhemoglobin (COHb), fetal hemoglobin (FHb), total hemoglobin (THb), and oxygen saturation (SaO2 [%]) in blood.
Hemoglobin, found in all animal blood, is the molecule responsible for collecting oxygen in the lungs and carrying it to the tissues. There, it releases the oxygen and picks up carbon dioxide. After returning to the lungs, the carbon dioxide is released and oxygen picked up once again. Hemoglobin production and loss can be affected by many conditions and explains why it is among the most commonly measured parameters in clinical medicine.
Blood total hemoglobin concentration ([THb]) is reported as grams of hemoglobin per deciliter of blood (g/dL). Patients frequently present either with previously unknown abnormalities in [THb], such as from iron deficiency anemia, or the [THb] becomes abnormal during their clinical course. For example, in the operating room, the change in [THb] can be rapid, difficult to identify, and hard to measure in a timely fashion. Examples of these situations include acute trauma with massive blood loss, large orthopedic surgical cases, and liver transplantation. In each of these examples, as well as many others, blood loss may be rapid, so that identifying the change in [THb] and correcting the abnormal [THb] quickly may be critical to patient survival.
Measurement of the various hemoglobin species concentrations in blood ([HbO2], [Hb], [metHb], [COHb]) allows an accurate assessment of blood oxygen content ([HbO2]×Vol) and oxygen saturation (SaO2=[HbO2]/([HbO2]+[Hb]+[metHb]+[COHb])×100%, or SpO2=[HbO2]/([HbO2]+[Hb])×100%. Carboxyhemoglobin concentration ([COHb]) can be dangerously elevated in tobacco smokers, house fire victims, firefighters, and attempted suicide victims. This condition does not reduce [THb] but effectively reduces the [HbO2] available for oxygen delivery to the tissues and can require aggressive intervention.
The current method of [THb] measurement is invasive and requires the drawing of a blood sample from the artery, vein, or fingertip of a patient, analysis of that sample by means of a laboratory test, and delivery of the subsequent result to the healthcare provider. This process is painful to awake patients and risks needle stick exposures to care providers. Furthermore, it is costly, time-consuming and distracting to the clinical care provider and can result in delayed patient care. Lastly, in a dynamic clinical situation, the patient's [THb] may continue changing between blood sampling and reporting so that critical decisions may be based on outdated information.
An accurate, clinically continuous noninvasive measurement of [THb], at a reasonable cost, would be a major step forward in healthcare technology. There would be widespread application for this device in the operating room and in a number of other clinical settings ranging from outpatient clinics to third-world countries where anemia screening is a major problem. It would thus be desirable to obtain fast and reliable measurements of the blood hemoglobin concentration through simple, noninvasive testing.